1. Field of the Invention
This invention relates broadly to tubular braided stents for use in a body vessel. More particularly, this invention relates to tubular braided stents with endovascular grafts for use in blood vessels.
2. State of the Art
Transluminal prostheses are well known in the medical arts for implantation in blood vessels, biliary ducts, or other similar organs of the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures or to support tubular structures that are being anastomosed. When biocompatible materials are used as a covering or lining for the stent, the prosthesis is called a stent-graft or endoluminal graft. If used specifically in blood vessels, the stent-graft is known as an endovascular graft. A stent may be introduced into the body by compressing or stretching it until its diameter is reduced sufficiently so that it can be fed into a catheter. The stent is delivered through the catheter to the site of deployment and then released from the catheter, whereupon it self-expands. The compression to expansion ratio and radial pressure of stents can usually be determined from basic braid equations. A thorough technical discussion of braid equations and the mechanical properties of stents is found in Jedweb, M. R. and Clerc, C. O., "A Study of the Geometrical and Mechanical Properties of a Self-Expanding Metallic Stent--Theory and Experiment", Journal of Applied Biomaterials; Vol. 4, pp. 77-85 (1993). In light of the above, it becomes evident that a stent must possess certain elastic and compression qualities.
A typical state of the art stent, such as disclosed in U.S. Pat. No. 4,655,771 to Wallsten or in U.K. Patent Number 1,205,743 to Didcott, is shown herein in prior art FIGS. 1, 1a, 2, and 2a. The typical stent 10 has a tubular body composed of wire elements 12, each of which extends in a helical configuration about the longitudinal axis 14 of the stent 10. Half of the elements 12 are wound in one direction while the other half are wound in an opposite direction. With this configuration, the diameter of the stent is changeable by axial movement of the ends 9, 11 of the stent. Typically, the crossing elements form a braid-like configuration having a specific pitch angle PA and are arranged so that the diameter of the stent 10 is normally expanded as shown in FIGS. 1 and 1a. Associated with the pitch angle PA is the pitch length PL. The pitch length is measured along the longitudinal axis 14 of the stent 10, and is defined as the distance it takes any wire element 12 of the stent 10 to complete one helical cycle. For example, and as illustrated in prior art FIG. 1a, wire element 32 of the stent 10 begins a helical cycle at the end 11 of the stent 10, and finishes it at line 34, giving the stent 10 a pitch length of PL, which is the distance as measured along the longitudinal axis 14 between the end 11 of the stent 10 and line 34. It will be appreciated from the above, that for any stent of the type illustrated in FIG. 1a, the pitch length PL of the stent is always an inverse function of the pitch angle PA of the stent (for pitch angles in the range 0.degree.-180.degree.). The diameter may be contracted bypulling the ends 11, 13 of the stent 10 away from each other as shown by the arrows 16, 18 in FIG. 2. When the ends of the body are released, the diameter of the stent 10 self-expands and draws the ends 11, 13 of the stent closer to each other. The degree with which the diameter may be contracted is dependent on the pitch angle PA of the crossing elements. Although the Wallsten and Didcott stent designs offer many improvements over the prior art, they still contain several disadvantages discussed in detail below.
Referring to prior art FIGS. 3a, 3b, 3c and 3d the design differences between the Wallsten and Didcott stents are shown. The Wallsten patent discloses a stent 300a (illustrated in FIGS. 3a and 3c herein) with crossing elements forming a pitch angle 301a in the 90.degree. to 160.degree. range. The advantage of these higher pitch angles is that they result in a higher radial force throughout the stent. The radial force is defined as the measure of the outward force that the stent exerts on the vessel into which it is deployed. For example, if a Wallsten type stent is made of thirty-six strands of 0.23 mm (0.009") diameter wire with a Young's modulus (Y=tensile stress/lengthwise strain) of 2.07 Mbar (30 Mpsi), a rigidity modulus of 0.93 Mbar (12 Mpsi) and a yield strength of 24.82 Kbar (360 Kpsi), the radial force outward according to standard braid equations would be approximately 30,000 Pascals when fully compressed.
Another advantage inherent in higher pitch angle stents is that the ends of a high pitch angle stent do not tend to narrow when placed in short aneurysmal necks. In particular, an aneurysm is a blood vessel which has undergone a permanent dilatation, usually caused by a weakening of the vessel wall. An aneurysm typically takes on a sac-like shape with an entrance neck and an exit neck which are connected to other non-aneurysmal blood vessels. When a stent is placed in an aneurysm 350 near a renal artery 398, as illustrated in FIGS. 3c and 3d, the stent takes on one of two different configurations depending on its braid pitch angle and pitch length; i.e., its "periodicity". A high pitch angle stent 300a (having conversely a short pitch length) such as seen in FIG. 3c, when placed in an aneurysm 350 expands evenly from one end 302a to the other 304a. As a result, the stent 300a is locked into place, and the necks 352, 354 of the aneurysm 350 are kept open by the pressure exerting ends 302a, 304a of the stent 300a. In particular, the ends 302a, 304a of the stent 300a are oriented away from the longitudinal axis 326, i.e., flared, and thus aid in affixing the stent 300a to the necks 352, 354 of the aneurysmal vessel 350. The affixed stent 300a is then prevented from moving down the vessel where it could potentially occlude side vessels or even perforate the vessel wall. It can be shown mathematically, that in order for a stent to seat evenly in an aneurysmal neck, the pitch length of the stent should be less than the diameter of the aneurysmal neck; i.e., to exhibit a desired periodicity effect. In general, this requirement often necessitates that the pitch angle of the stent be greater than 160.degree..
The Wallsten type stent with the larger pitch angle, however, suffers from several shortcomings. Due to the high pitch angle and associated radial force, the Wallsten stent undergoes a large axial elongation upon radial compression. For example, a Wallsten stent with a 160.degree. pitch angle will undergo a 467% elongation upon an 82% diameter contraction. In other words, in order to place a 10 cm long stent of the Wallsten design having a 25 mm diameter in a 25 mm vessel with a catheter having an internal diameter of 4.5 mm, would require the catheter to be approximately 0.5 meter in length. Placement of a stent with this tremendous elongation and radial force is very difficult for several reasons. First, the stent would have to be pushed out of the catheter over a very long distance, which may be extremely difficult in light of the increased friction forces and various bent sections encountered in the catheter as it traverses a tortuous path. Second, the stent will shrink significantly in length as its diameter expands, thereby rendering it difficult to accurately place it in a vessel. The importance of extreme accuracy in placement of an endovascular graft will be appreciated by those knowledgeable in the art. For example, in aneurysmal vessel disease, such as that encountered in the abdominal aorta where the distance between the renal arteries and the aneurysm is quite short (less than 3 cm), misplacement of an endovascular graft over the renal arteries can prove fatal. Similarly, misplacement of the stent openings in the aneurysm can also prove detrimental. Another disadvantage of the Wallsten design, is that the coating material which provides the graft component of the endovascular graft must be able to elongate with the stent several times (e.g. 4 to 5 times) its normal length. There are few graft materials that can undergo this extent of elongation without tearing. In practice, virtually all of the stents made according to Wallsten must have an angle of less than 120.degree., simply to facilitate placement in the body and minimize the aforementioned problems. Yet another disadvantage of the Wallsten design is that in bridging aneursyms with endoluminal grafts, longitudinal pressure is important in order to seat the graft in the necks of the aneurysm. However, stents with a high pitch angle such as Wallsten do not provide large longitudinal pressures.
The Didcott stent 300b (illustrated in FIGS. 3b and 3d herein), on the other hand, does not have the excessive elongation to contraction ratio associated with the Wallsten stent. Due to the suggested acute pitch angle 301b associated with the Didcott stent, the Didcott stent has the advantage of undergoing only a relatively small axial extension when contracted for introduction through a catheter. For example, for a stent of the same design and having the same dimensions as the Wallsten stent above, but made with a pitch angle of 90.degree., the Didcott stent will contract from a 25 mm diameter to a fit in a catheter having an internal diameter of approximately 4.5 mm with a resulting axial elongation of only 4 cm. Thus, the stent undergoes only a 40% elongation upon a 82% diameter contraction. The advantage of smaller elongation, however, comes with the disadvantage of providing a significantly weaker radial force than the Wallsten stent. For the same thirty-six strands of 0.23 mm diameter (0.009") wire described above, the Didcott stent has a radial force when fully compressed of only 8,000 pascals.
Another disadvantage of the Didcott low pitch angle stent design is that the ends 302b, 304b of the stent 300b have more of a tendency to perforate the arterial wall of an artery into which the stent is placed because the end wires of the stent 300b are pointed more towards the longitudinal axis of the stent with a slight flare 306b to the outside. With such an arrangement, the hydraulic force of the passing blood flow tends to force the stent downstream, pushing the sharp ends of the stent wires into the blood vessel wall. If enough wire is forced into the vessel wall, the wall can perforate. Yet another disadvantage of the Didcott design relates to when the stent is placed in an aneurysm with a short neck. Because of the periodicity of Didcott, i.e., a low pitch angle and a large pitch length, the Didcott stent undesirably tends to close in the aneurysm neck.
Additional disadvantages are shared by all prior art stents, including both the Didcott and the Wallsten stents. For prior art stents which all have substantially homogeneous pitch angles, the radial load or force (the radial load is proportional to the radial pressure, which is a measure of the outward force that the stent exerts on the vessel into which it is deployed) in the middle of the stent is always greater than the radial force or load at the ends of the stent. This inequality is due to the fact that the stent wires in the middle of the stent are supported on both sides while the wires on the end of the stent only have one side for support. For example, a stent made with 36 wires of 0.009" Elgiloy braided at a homogeneous pitch angle of 75.degree. and with a 25 mm diameter has a radial load in the center of the stent of 0.62 lb. and at the ends of the stent of 0.14 lb. when at 50 percent compression. This discrepancy in radial force can cause the stent to take on a cigar or football-like shape in a vessel. The ensuing constriction of the ends of the stent can subsequently lead to occlusion of the stent, especially in small diameter vessels where any change in cross-sectional area is very significant. Furthermore, when dealing with an aneurysm, a stent cannot be anchored to the walls of a vessel by its body section as the vessel walls are either damaged and thin or missing. Anchoring at the ends would be equally unfeasible in light of the above described discrepancy in radial force and resulting cigar-like shape of the stent. Thus, the prior art stents cannot advantageously be used in those situations where the stent must be locked in a particular position and the walls of the body cavity in which the stent is inserted cannot provide anchoring means.